Support GET status of resumable uploads

[amouat]

This seems to be missing from Trow:

https://github.com/opencontainers/distribution-spec/blob/master/detail.md#blob-upload

But to be honest I'm not sure the whole resumable upload thing actually works properly anywhere. It seems to be only half implemented in Docker Distribution, and only as of late 2019: https://github.com/docker/distribution/blob/6c72ec2e8514dbe3a96506316f80351ed95313a1/registry/handlers/blobupload.go#L136 . For this reason, I've only labelled it enhancement.

We should still try to implement it, going by the spec, but even that seems a little inconsistent about what headers are returned when and in what format e.g. should it start "bytes="? Shouldn't it be symmetrical with the fetch blob part calls (which we also don't implement...).

We also need proper tests for all this.